Removal of I, Rn, Xe and Kr from off gas streams using PTFE membranes

ABSTRACT

A process for removing I, R, Xe and Kr which involves the passage of the off gas stream through a tube-in-shell assembly, whereby the tubing is a PTFE membrane which permits the selective passages of the gases for removing and isolating the gases.

CONTRACTUAL ORIGIN OF THE INVENTION

The U.S. Government has rights in this invention pursuant to ContractNo. DE AC07-79-ID01675 between the U.S. Department of Energy and ExxonNuclear Idaho Company.

BACKGROUND OF THE INVENTION

This invention relates to countercurrent extraction methods and inparticular, to a means for separating gaseous elements from the liquidphase components of a waste gas stream.

Removal of gaseous radioactive iodines and other elemental species froma waste gas stream to prevent their entrance to the atmosphere is ofconsiderable importance. It is primarily by such means that one canprevent the contamination of surfaces to which these substances mayadhere and also to prevent ingestion by humans and animal life. Theradioactive gaseous elements of concern here can include inorganicspecies such as various elemental substances, as well as organic speciessuch as methyliodide which may be present in waste gases which maycomprise radioactive off gases from nuclear fuel reprocessing plants.These radioactive materials are formed in the nuclear reactor fuel bythe fission of the fuel material. If these off gases can be individuallycollected from the gaseous effluent, they might possibly be used forother unrelated purposes.

Various means have been used in the past for filtering out and/ortrapping such radioactive materials in the off gas stream, for example,materials such as copper and other metals which react with iodine havebeen used to adsorb the iodine that is not stopped by mechanicalfiltration. Silver nitrate supported on an inert substrate reacts withthe iodine species to form silver iodide. Another material commonly usedin cleanup systems of reactor containment atmospheres for fissionproducts is charcoal impregnated with iodine and potassium iodide.Various inefficiencies and limitations in the use of these differentmeans for separating such effluent gaseous mixtures into their componentgases have indicated the need for a more versatile and more efficientmeans of separating the individual gases from the effluent.

SUMMARY OF THE INVENTION

Therefore an object of the subject invention is a method for adsorbingand retaining both inorganic and organic radioactive gaseous species.

Another object of the subject invention is a method for separatingradioactive gaseous materials from a gaseous effluent while simplifyingthe use of cryogenic methods.

Still another object of the subject invention is a method for separatingiodine, xenon, krypton and radon from a gaseous effluent in the presenceof oxides of nitrogen.

These and other objects are attained through the use of the subjectinvention whereby the gaseous elements of iodine, xenon, krypton andradon may be individually separated from a gaseous effluent even thoughnitrogen oxides are present, by passing the gaseous effluent into atube-in-shell countercurrent mass transfer apparatus through tubingformed of expanded microporous, polytetrafluoroethylene (PTFE) tubing.An aqueous acceptor stream flowing in the shell of the firstcountercurrent exchange apparatus receives substantially all of thetrace level gaseous components present in the original gas stream. Onlya saturation-limited fraction of the bulk carrier gases, N₂ and H₂, in atypical off gas stream enters the aqueous acceptor stream, so a grossseparation between the carrier gases and the trace gases occurs in thefirst exchanger. This aqueous stream then becomes the donor stream whilepassing through the second countercurrent exchange apparatus. Themajority of the NO₂ dissolved in the water reacts to form involatile(ionic) nitric and nitrous acids which are discarded along with thewater after it passes through the second exchanger. The second exchangertransfers the remaining gases, (radon, xenon, krypton, iodine, and someNO) to a recirculating nitrogen "extractant" gas stream pumped betweenthe second and third exchangers.

In the third exchanger, the trace-level gases are transferred to arecirculating stream of an aqueous reductant, such as a dilute sodiumsulfite solution which is pumped through the third and fourthexchangers. The sulfite reduces the iodine to nonvolatile iodide ionwhich makes the solution act as a sink for that element.

A nitrogen gas acceptor stream in the final exchanger is recirculatedthrough a cryogenic separation system which, as known in the art,recovers the remaining valuable radioaotive inert gas components(radon/krypton/xenon) for eventual sale as byproducts.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic showing the countercurrent extraction of thegaseous elements from an effluent according to the subject invention.

DETAILED DESCRIPTION OF THE EMBODIMENT

Referring now to the drawing, there is shown a countercurrent extractionapparatus comprised of four tube-in-shell assemblies 1, 2, 3, and 4,each of which may be anywhere from one to thirty meters long. In actualindustrial practice, each of these exchangers may be comprised of asmany paralleled individual tube-in-shell exchangers as needed to providethe necessary gas handling capacity. Each individual tube-in-shell gasexchange assembly (1, 2, 3, and 4) has an impermeablepolytetrafluoroethylene (PTFE) tubing shell of approximately 2 mm IDsurrounding a microporous PTFE tube having an outside diameter ofapproximately 1.5 mm. The inner tube is permeable to all gases but doesnot permit the passage of aqueous liquids. A microporous PTFE tubinghaving a porosity of 30% to 70% with a pore size of approximately 1 to3.5 μm have been found suitable for use with the subject invention.Preferably, microporous PTFE tubing with a 1 mm ID, wall thickness of0.4 mm and a porosity of 50% with a maximum pore size of 2.0 μm has beenfound to work best. Tubing suitable for this application is availableunder the trademark, "Goretex" from W. L. Gore and Associates, Inc.,Elkton, Md.

In the first tube-in-shell apparatus 1, the off gas effluent [typicallylargely comprised of H₂ and N₂, much smaller amounts of nitrogen oxides(NO and/or NO₂ at concentrations ranging from virtually zero to a fewpercent of the gas stream) and trace levels of radioactive iodine andinert gases] passes up the first porous tube 5 while a stream of purewater passes countercurrently down shell 9. Virtually all of the tracegases and the nitrogen oxides transfer to the aqueous stream through theporous wall of tube 5. However, the bulk of the N₂ and H₂ is nottransferred to the aqueous stream because of their finite solubility inthe limited volume of the aqueous phase. The absorbed NO₂ reacts withthe water to form nitric acid and nitrous acid. Some fraction of thenitrous acid may then disproportionate to form more nitric acid and NOgas at a stoichiometric ratio of approximately 1:2.

The aqueous stream then passes up tube 6 in the second tube-in-shellexchanger 2 and a countercurrent flow of an extractant gas (e.g.,nitrogen, helium, or hydrogen) is passed down the shell 10 of the sameexchanger. That fraction of the NO₂ originally absorbed by the waterflow and which reacted to form nitric acid is discarded along with thewater at the raffinate exit at the top of tube 6.

The volatile iodine, NO, and the inert gases dissolved in that solutionare quantitatively transferred to the extractant gas stream which isitself continuously looped/recycled between shell 10 of exchanger 2 andtube 7 of exchanger 3. In the third exchanger, the gases are transferredfrom the extractant gas stream to a aqueous stream containing areductant such as sodium sulfite. The reductant is pumped in acontinuous loop comprised of the shell 11 of exchanger 3 and the tube 8of exchanger 4 and forms a sink for the iodine, which is reduced tononvolatile iodide ion by the reductant solution. The reductant maycomprise a sodium sulfite solution as stated; sodium bisulfite or sodiummetabisulfite may also be used. Periodically a portion of the aqueousstream can be removed and the radioactive iodide precipitated withsilver nitrate to form a more compact and chemically stable waste form.

In exchanger 4, the inert gases and NO in the reductant solution aretransferred to another extractant gas stream which is continuouslycycled between shell 12 of exchanger 4 and a cryogenic separation system13 preceded by a rhodium catalyst bed 14. The catalyst bed 14 decomposesNO to elemental oxygen and elemental nitrogen and the cryogenicseparation system separates the valuable radioactive inert gases fromthe nitrogen, oxygen, and water vapor. The cryogenic separator used maybe any of those commonly available for separating gases through exposureto variable low temperatures. One such apparatus is the "Rare Gas Plant"at the Idaho National Engineering Laboratory. The rhodium catalyst bedcomprises a bed of rhodium-coated alumina, although any other means ofseparating the NO from the gas stream may be used. Other means ofseparating the gases and decomposing the NO may be utilized as well.

When sufficient radioactive iodine has accumulated in the aqueousreductant stream, a portion of the solution can be removed and theiodine removed by precipitation with a silver salt such as AgNO₃ or thelike and thereby converted to a stable and compact solid waste form.

While the invention has been described with reference to a preferredembodiment, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment disclosed as the best modecontemplated for carrying out this invention, by that the invention willinclude all embodiments and equivalents falling within the scope of theappended claims.

Various features of the invention are set forth in the following claims.

We claim:
 1. A process for separating gaseous elements including iodine,xenon, krypton, and radon from a gaseous effluent in the presence ofoxides of nitrogen comprising:passing said gaseous effluent across agas-permeable membrane into an aqueous flow to transfer said elementsand said oxides of nitrogen into said aqueous flow; passing said aqueousflow across a second gas-permeable membrane against a flow of a firstextractant gas to selectively transfer the gaseous elements to saidfirst extractant gas; flowing said first extractant gas with thetransferred gaseous elements across a third gas-permeable membrane intoa flow of reductant solution to transfer said gaseous elements and toconvert iodine to an iodide; flowing said reductant solution with saidgaseous elements and said iodide across a fourth gas-permeable membraneinto a second extractant gas to transfer xenon, radon, and krypton intosaid second extractant gas; and cryogenicly separating said xenon,radon, and krypton from said second extractant gas.
 2. The process ofclaim 1 wherein said first extractant gas is selected from the groupconsisting of hydrogen, helium, and nitrogen.
 3. The process of claim 1wherein said reductant solution is selected from the group consisting ofsodium sulfite, sodium bisulfite, and sodium metabisulfite.
 4. Theprocess of claim 1 wherein said membranes each comprise microporouspolytetrafluoroethylene.
 5. The process of claim 4 wherein saidmembranes have a porosity of 30% to 70% and a pore size of about 1 μm to3.5 μm.
 6. The process of claim 1 wherein said first extractant gascomprises hydrogen.
 7. The process of claim 1 wherein said firstextractant gas comprises helium.
 8. The process of claim 1 wherein acatalytic bed converts NO to elemental nitrogen and elemental oxygenprior to the cryogenic separation.
 9. A process for separating gaseouselements including iodine, xenon, krypton, and radon from a gaseouseffluent in the presence of oxides of nitrogen comprising:passing saidgaseous effluent across a gas-permeable membrane of microporouspolytetrafluoroethylene into an aqueous flow to transfer said elementsand said oxides of nitrogen into said aqueous flow; passing said aqueousflow across a second gas-permeable membrane of microporouspolytetrafluoroethylene against a flow of a first extractant gasselected from the group consisting of helium, hydrogen, and nitrogen toselectively transfer the gaseous elements to said first extractant gas;flowing said first extractant gas with the gaseous elements across athird gas-permeable membrane of microporous polytetrafluoroethylene intoa flow of sodium sulfite solution to transfer the gaseous elements andto convert the iodine to the iodide; flowing said sulfite solution withthe gaseous elements and the iodide across a fourth gas-permeablemembrane into a second extractant gas; and cryogenicly separating saidxenon, radon, and krypton from said second extractant gas.